The present invention concerns a pretreatment method useful for increasing the conversion and lowering the fouling rate of a reforming catalyst.
Catalytic reforming is a well-known process that is used for raising the octane rating of a naphtha for gasoline. The reactions that occur during reforming include: dehydrogenation of cyclohexanes, dehydroisomerization of alkylcyclopentanes, dehydrocyclization of acyclic hydrocarbons, dealkylation of alkylbenzenes, isomerization of paraffins, and hydrocracking of paraffins. The hydrocracking reaction should be suppressed because that reaction lowers the yield of hydrogen and lowers the yield of liquid products.
Reforming catalysts must be selective for dehydrocyclization, in order to produce high yields of liquid product and low yields of light gases. These catalysts should possess good activity, so that low temperatures can be used in the reformer. Also, they should possess good stability, so that they can maintain a high activity and a high selectivity for dehydrocyclization over a long period of time.
While most reforming catalysts contain platinum on an alumina support, large-pore zeolites have been proposed as supports. These large-pore zeolites have pores large enough for hydrocarbons in the gasoline boiling range to pass through. Commercial application of zeolitic reforming catalysts have thus far been very limited, although certain catalysts comprising a large-pore zeolite containing at least one Group VIII metal have a very high selectivity for dehydrocyclization.
It is known that reforming catalysts require pretreatment prior to utilizing these catalysts for reforming naphtha feedstocks. For example, U.S. Pat. No. 4,517,306 issued to Waldeen Buss on May 14, 1985 claims a composition comprising: (a) a type L zeolite; (b) at least one Group VIII metal; and (c) an alkaline earth metal selected from the group consisting of barium, strontium and calcium, wherein said composition is reduced in a hydrogen atmosphere at a temperature of from 480.degree. C. to 620.degree. C. (896.degree. to 1148.degree. F). It is preferred that the composition be reduced at a temperature from 550.degree. to 620.degree. C. (1022.degree. to 1148.degree. F.).
U.S. Pat. No. 4,539,304 issued on Sep. 3, 1985 to Field discloses a two-step pretreatment process for increasing the conversion of reforming catalysts wherein the catalyst is first treated at a temperature of from 120.degree. C. (248.degree. F.) to 260.degree. C. (500.degree. F.) in a reducing gas. In the second step, the temperature of the catalyst is maintained at 370.degree. C. (698.degree. F.) to 600.degree. C. (1112.degree. F.) in a reducing atmosphere.
U.S. Pat. No. 4,539,305 issued on Sep. 3, 1985 to Wilson et al. discloses a pretreatment process for enhancing the selectivity and increasing the stability of a reforming catalyst comprising a large-pore zeolite containing at least one Group VIII metal. The catalyst is reduced in a reducing atmosphere at a temperature of from 250.degree. C. (482.degree.) to 650.degree. (1202.degree. F.). The reduced catalyst is subsequently exposed to an oxygen-containing gas and then treated in a reducing atmosphere at a temperature of from 120.degree. C. (248.degree. F.) to 260.degree. C. (500.degree. F.). Finally, the catalyst is maintained at a temperature of from 370.degree. C. (698.degree. F.) to 600.degree. C. (1112.degree. F.) in a reducing atmosphere. Preferably, the first reduction step is carried out in the presence of hydrogen.
U.S. Pat. No. 5,155,075 issued to Innes et al. shows an initial catalyst reduction at 300.degree. F. to 700.degree. F., followed by a temperature ramp up to a final hydrogen treatment temperature between 900.degree. F. and 1000.degree. F.
U.S. Pat. No. 5,066,632 issued on Nov. 19, 1991 to Baird et al. discloses a process for pretreating a catalyst useful for reforming a naphtha wherein the catalyst is calcined at temperatures in excess of 500.degree. F., preferably at temperatures ranging from 500.degree. F. to about 750.degree. F. in air or in atmospheres containing low partial pressures of oxygen or in a non-reactive or inert gas such as nitrogen. The catalyst is then contacted with a dry hydrogen-containing gas at a temperature ranging from about 600.degree. F. to about 1000.degree. F., preferably from about 750.degree. F. to about 950.degree. F., at a hydrogen partial pressure ranging from about 1 atmosphere to about 40 atmospheres, preferably from 5 atmospheres to about 30 atmospheres.
European Patent Application Publication Number 4 243,129 discloses a catalyst activation treatment with hydrogen at temperatures from 400.degree. C. (752.degree. F.) to 800.degree. C. (1472.degree. F.), preferably from 400.degree. C. (752.degree. F.) to 700.degree. C. (1292.degree. F.), for a catalyst used for cracking a hydrocarbon feedstock. The treatment pressure may vary from 100 to 5,000 MPa but is preferably from 100 to 2,000 MPa. A carrier gas which contains 1-100% v/v, preferably from 30-100% v/v, of hydrogen is used.
U.S. Pat. No. 4,717,700 issued to Venkatram et al discloses a method for drying a zeolite catalyst by heating while in contact with a gas. The rate of catalyst temperature increase is controlled so as to limit the rate of water evolution from the catalyst and the water vapor concentration in the gas. The gas used to heat the catalyst is gradually increased in temperature at about 28.degree. C. per hour. The moisture level of the effluent gas is preferably between 500 and 1500 ppm during the drying step. The catalyst drying method with a subsequent reduction with hydrogen wherein the temperature is raised to a maximum temperature of 450.degree. C. is exemplified in Example 1.
Austrian Patent Specification No. 268,210 relates to a metal-charged zeolite molecular sieve, which is suitable as a catalyst for the conversion of hydrocarbons. Methods for preparing the catalyst are described. It is disclosed that the catalyst prepared by such methods usually has a high water content and that it is desirable to activate the catalyst before use since the catalyst is sensitive to water. The recommended activation process comprises: 1) slow heating of the catalyst in air at 300.degree. to 600.degree. C., preferably 500.degree. C.; followed by 2) slow heating of the catalyst from room temperature to approximately 500.degree. C. in a current of hydrogen gas under atmospheric pressure.
A pretreatment process of Pt-Al.sub.2 O.sub.3 catalysts in hydrogen in the temperature range of 450.degree. C. (842.degree. F.) to 600.degree. C. (1112.degree. F.) is disclosed in Journal of Catalysis (1979); Vol. 59, p. 138 (P. G. Menon and G. F. Froment). The effect of catalyst reduction temperature on the conversion of n-pentane and n-hexane using Pt-Al.sub.2 O.sub.3 catalysts is disclosed. For the Pt-Al.sub.2 O.sub.3 catalyst reduced at 400.degree. C. (752.degree. F.), hydrogenolysis is the main reaction; whereas for the Pt-Al.sub.2 O.sub.3 catalyst reduced at 600.degree. C. (1112.degree.), the hydrogenolysis and total activity are considerably suppressed. This reference specifically discloses the effect of a hydrogen pretreatment process on Pt-Al.sub.2 O.sub.3 catalysts and does not disclose the effect of hydrogen pretreatment on zeolitic catalyst.
Additionally, the effects of hydrogen pretreatment of the Pt-Al.sub.2 O.sub.3 catalyst with respect to isomerization is disclosed. The activity for dehydrocyclization was not increased.
Prior art processes have observed both a reduced catalytic activity and reduced hydrogen chemisorption for catalysts which have been reduced at temperatures in excess of 500.degree. C. Furthermore, there has been no clear understanding of the phenomena which occur during high temperature catalyst reduction. Thus, reduction at high temperatures may result in strongly chemisorbed hydrogen, may cause loss of spillover hydrogen altering the local charge transfer from the support to the metal at the particle boundary, may induce changes in morphology of the metal crystallite, or may affect reduction of the support resulting in the formation of an alloy with atoms from the support.